In recent years, semiconductor devices having a trench gate structure have attracted attention. While a channel is formed on the surface of a semiconductor layer in semiconductor devices having a planar gate structure, a channel region is formed on the side of a trench provided in a semiconductor layer in the semiconductor devices having the trench gate structure. The trench gate structure semiconductor devices are therefore more expected to achieve miniaturization and reduction in ON resistance than the planar gate structure semiconductor devices. For this reason, in the field of power devices, in particular, development of trench gate structure semiconductor devices is underway.
The trench gate structure semiconductor devices, which are free from the limitation on miniaturization caused by the junction FET (JFET) effect, have an advantage that a fine trench can be formed to permit reduction in ON resistance and switching loss. However, a problem arises that the aspect ratio of a fine trench is large, making it difficult to embed a gate electrode in such a trench. Also, since the cross-sectional area of the gate electrode decreases, the gate resistance will increase. To prevent or reduce the increase in gate resistance, it is being examined to form a gate electrode in a T shape to extend the gate electrode over the periphery of the trench (see Patent Document 1, for example).
Also, in the trench gate structure semiconductor devices, it is important to form a suitable gate insulating film inside the trench. An electric field is concentrated on the bottom of the trench, where the field strength is higher than in any other portion. Therefore, there is the possibility that a breakdown due to a dielectric breakdown of the gate insulating film may occur at the bottom of the trench.
If the gate insulating film is thickened as a whole to increase the dielectric breakdown field, the threshold voltage at switching will increase. Therefore, a method of forming a thicker gate insulating film on the bottom of the trench using a difference in plane direction between the side and bottom surfaces of the trench is being examined (see Patent Document 2, for example). Also being examined is a method where a mask is formed on the side of the trench during formation of the gate insulating film to form a thick gate insulating film on the bottom of the trench while preventing or reducing formation of an oxide film on the side of the trench (see Patent Document 3, for example).